1. Field of the Invention
The present invention relates to a method of forming a critical resist pattern, and more particularly to a method of forming a critical resist pattern improved so as to form a resist pattern having an accurate rectangular sectional configuration and high dimension controllability.
2. Description of the Background Art
In the manufacturing process of a device equal to or greater than 64MDRAM, there is the need of development of quarter micron lithography. In the current lithography process using deep UV light from an excimer laser and the like, a 3 component chemical amplification positive type resist is employed including an acid generating agent decomposed by photochemical reaction to generate acid, a base resin including a functional group having low absorption with respect to deep UV light and that decomposes by acid catalyst, and in which solubility with respect to alkali developing solution is increased at the light irradiated area, and a dissolution inhibitor. Also, a 2 component chemical amplification positive type resist not including dissolution inhibitor is employed.
FIG. 7 shows the chemical structural formula of a base resin which is the structural component of a 2 component chemical amplification positive type resist (SPIE Vol. 1086 Advances in Resist Technology and Processing VI (1989)). Referring to FIG. 7, it is appreciated that the base resin is a poly (p-t-butoxycarbonyl-oxy-styrene) having t-butyl oxycarbonyl (referred to as t-BOC group hereinafter) coupled to poly-p-hydroxy styrene, where n is a natural number representing the polymerization degree. The acid generating agent shown in FIG. 8(a) is triphenyl sulfonium hexafluoro antimonate. As shown in FIG. 8(b), triphenyl sulfonium hexafluoro antimonate generates protonic acid by being subjected to light irradiation.
A conventional method of forming a resist pattern using a chemical amplification positive type resist including these base resin and acid generating agent will be described with reference to FIG. 6.
Referring to FIG. 6(a), a chemical amplification positive type resist 3 for excimer laser lithography is spin-coated to a film thickness of approximately 1.0 .mu.ml-5 .mu.m on a semiconductor substrate 2. Then, this is softbaked at a temperature within the range of 80.degree. C.-130.degree. C.
Referring to FIG. 6(b), excimer laser light 8 for forming an image is selectively directed to chemical amplification positive type resist layer 3 via a reticle 9. The exposed portion 5 of chemical amplification positive type resist 3 has triphenyl sulfonium hexafluoro antimonate decomposed to generate protonic acid 4, as shown in FIG. 8.
Referring to FIG. 6(c), semiconductor substrate 2 is mounted on a hot plate 10, whereby chemical amplification positive type resist layer 3 is baked for 1-2 minutes at a temperature of 60.degree. C.-100.degree. C. This process is called post exposure baking (referred to as PEB hereinafter). This PEB causes the t-BOC group in the base resin to be released in the exposed portion 5, whereby solubility of the base resin with respect to the alkali developing solution is increased.
The release of the t-BOC group in the base resin by acid catalysis will be described in details with reference to FIG. 9.
Referring to FIG. 9(a), excimer laser light is directed to chemical amplification positive type resist layer 3. This causes triphenyl sulfonium hexafluoro antimonate in chemical amplification positive type resist layer 3 to be decomposed to generate protonic acid, as shown in FIG. 9(b).
Referring to FIG. 9(c), chemical amplification positive type resist layer 3 is baked, whereby t-BOC functional group is released from poly (p-t-butoxycarbonyl-oxy-styrene) to generate hydroxyl group according to the reaction formula shown in FIG. 10. Continuation of this baking process promotes the release of the t-BOC functional group as shown in FIG. 9(d). Finally, the base resin is converted to poly vinyl phenol that is soluble in an alkali developing solution, as shown in FIG. 9(e).
FIG. 11 shows the difference in the dissolution rate between poly (p-t-butoxycarbonyl-oxy-styrene) and poly (p-hydroxy styrene), i.e. the difference of dissolution rate between the exposed portion and the not-exposed portion of a chemical amplification positive type resist with respect to an alkali developing solution. It is appreciated from FIG. 11 that the dissolution rate with respect to an alkali developing solution in the exposed portion of the chemical amplification positive type resist increases.
By eluting the exposed portion 5 of resist film 3 with an alkali developing solution of an appropriate concentration, a resist pattern 7 of high sensitivity and high resolution is obtained.
Thus, high sensitivity and high resolution can be achieved with excimer laser lithography employing a conventional chemical amplification positive type resist.
However, exposure of a chemical amplification positive type resist layer is carried out in a clean room. The atmosphere in a clean room includes oxygen and a small amount of basic components (ammonia, amine and the like from the developing solution). Therefore, at the surface of a chemical amplification positive type resist layer that is in contact with the atmosphere in the clean room, acid generated by deep UV light irradiation and reaction intermediates generated during acid generation will react with oxygen and basic components in the atmosphere. This produced a problem that solubility with respect to an alkali developing solution is significantly reduced at the surface of the chemical amplification positive type resist layer.
This problem will be described in detail with reference to FIG. 12.
FIG. 12(a) has the distance between the surface of a resist and a substrate (thickness of the resist) plotted along the abscissa and the acid concentration (arbitrary unit) plotted along the ordinate. FIG. 12(b) has the distance between the surface of a resist to a substrate (thickness of the resist) plotted along the abscissa and dissolution rate with respect to an alkali developing solution is plotted along the ordinate.
It can be appreciated from FIGS. 12(a) and 12(b) that an exposed chemical amplification positive type resist left in a clean room will exhibit reduction in acid concentration in the portion of thickness .DELTA.x at the surface layer of the resist. This means that solubility with respect to an alkali developing solution is reduced in the portion of thickness .DELTA.x. .DELTA.x is 200-500.ANG..
Referring to FIG. 12(c), an insoluble layer is formed at the portion of thickness .DELTA.x at the surface layer of resist pattern 7, resulting in the generation of a projecting corner 7a at the top portion of resist pattern 7. The generation of a projecting corner 7a will prevent the provision of a resist pattern having an accurate rectangular sectional configuration and high dimension controllability.